Director of the U.S. Centers for Disease Control visited Sierra Leone Monday to assess the fight against Ebola, which has killed more than 11,000 people in West Africa.

Stopping in Freetown, Dr. Thomas Frieden expressed optimism the deadly virus can be stamped out, but insists Sierra Leone needs to continue keeping its guard up, much as it did when a recent Ebola death caused 500 people to be put in quarantine in the country's Northern Tonkolili District, which had previously gone 150 days with no Ebola cases.

"The response has been robust, quick, there was identification of contacts, monitoring of contacts and several contacts become ill and they were tested and isolated," he said. "That’s how you stop outbreaks."

Frieden says the main reasons for ongoing cases are unsafe burials and unsafe caregiving in private homes instead of at Ebola treatment centers.

Citing the recent World Health Organization announcement of successful Ebola vaccine trials in Guinea, Frieden says CDC officials will work with the government of Sierra Leone and the WHO to determine whether more vaccinations should occur in Sierra Leone, and for whom.

“There is something called a Data Safety Monitoring Board, and they are meeting today to determine whether there is some difference that should be made in how the trial is undertaken [in Sierra Leone]” he said.

For taxi driver Mohamed Barrie, whose business has suffered because of Ebola, the news provides a sense of hope.

“Many of my friends lost their business and I lost two of my friends [to Ebola]," he said. "So it has affected people a lot.”

Frieden says the CDC will continue its work improving health systems in Sierra Leone, Guinea and Liberia and conduct further investigation into post Ebola symptoms survivors face, possibly by creating a national public health institute in Sierra Leone.